1. Field of the Invention
The present invention relates to an optical pick-up apparatus. More particularly, the present invention relates to a diffraction element and an optical pick-up apparatus having the same.
2. Description of the Related Art
An optical pick-up apparatus is employed in a CDP (Compact Disc Player), a DVDP (Digital Versatile Disc Player), a CD-ROM driver, or the like to perform recording and reproducing of information of a disc, which is an optical medium, in a contactless manner. When recording information, such an optical pick-up apparatus illuminates a laser beam onto a surface of a disc to form pits. When reproducing recorded information, the optical pick-up apparatus optically reads out pit information formed on the disc and outputs the information in an electric signal. To record information on and reproduce information from a disc in this manner, the optical pick-up apparatus includes a laser diode, which is a light source for illuminating a laser beam; at least one diffraction element for splitting the beam projected from the light source into three beams; a beam splitter for controlling the deviation of the laser beam; plural lenses for forming an optical path; and plural optical devices, such as an optical detector, for detecting a signal.
Such an optical pick-up apparatus performs focusing control for controlling an objective lens in a vertical direction to align a beam spot onto a surface of a disc and tracking control for controlling the objective lens in a horizontal direction to make a beam follow a track on the disc. To perform the focusing control and the tracking control, it is necessary to produce a focus error signal (hereinafter, referred to as “FE signal”) and a tracking error signal (hereinafter, referred to as “TE signal”). To produce an FE signal, an astigmatism method is usually employed. To produce a TE signal, although a push pull (hereinafter, referred to as “PP”) method has been employed, a differential push pull (hereinafter, referred to as “DPP) method, in which three beams are used, has recently been employed. When the PP method is employed, a DC offset is produced in the TE signal due to objective lens shift and tilt caused as the tracking servo is performed, thereby causing an error in the TE signal.
The DPP method calculates a value of a signal by using a main beam illuminated to the center of a track and two sub-beams spaced by a predetermined distance from the main beam in radial and tangential directions, respectively, and illuminated around a groove. The DDP method has an advantage in that a DC offset produced in a TE signal by the objective lens shift and tilt can be cancelled. However, it is not possible to employ such a DPP method for discs different from each other in specification of track pitch because illumination positions of sub-beams are varied in such discs.
For this reason, a method illuminating three beams onto a track has been recently developed and employed, an example of which method is illustrated in FIGS. 1A to 1C. Referring to FIGS. 1A and 1B, a diffraction element 10 is divided into two regions 12 and 14, each being formed with a grid pattern in which grids are formed with a predetermined pitch P. The grid patterns respectively formed in the first and second regions are arranged to be deviated by a half pitch (½P) from one another to produce a phase difference in each sub-beam SBF and SBB before the sub-beams are incident onto a disc, wherein such a phase difference is produced when an existing sub-beam is illuminated onto a groove and a main beam is illuminated onto a track. A beam incident onto the diffraction element 10 configured as described above is diffracted and split into one main beam (MB), which is a zero order diffracted beam, and two sub-beams SBF and SBB, which are ±1st order diffracted beams, respectively, and of which one trails the other on a track T of a disc D. Additionally, the two sub-beams SBF and SBB are diffracted and split into two sub-beams SBF1 and SBF2, and SBB1 and SBB2, respectively, due to the separation of the first region 12 and the second region 14. The main beam MB and the sub-beams SBF and SBB diffracted by the diffraction element 10 are incident onto the disc D, and the beams incident onto the disc D are reflected and then received by an optical detector 20 as shown in FIG. 1C. Then, the optical detector 20 outputs a signal depending on the light intensity detected on each of the divided regions, whereby a TE signal and a FE signal are calculated. The FE signal is calculated through a differential astigmatism method that employs the astigmatism of a beam reflected from the disc D. According to the differential astigmatism method, the FE signal is calculated in such a manner of summing differences in signal values determined from respective diagonal regions of an optical detection element 20a for use in detecting a main beam and optical detection elements 20b and 20c, each for use in detecting a sub-beam.
However, as shown in FIG. IC, the two sub-beams SBF1 and SBB1, and SBF2 and SBB2, which correspond to the first region 12 and the second region 14 in the diffraction element 10, respectively, are received by the optical detection elements 20b and 20c in a state in which the two sub-beams are spaced and separated by a predetermined distance from each other. In this manner, because the two sub-beams SBF1 and SBB1, and SBF2 and SBB2 are spaced and separated from each other on the optical detection elements 20b and 20c by a predetermined distance, the difference in signal values in the diagonal regions of the optical detection elements 20b and 20c, (E1+E3)−(E2+E4) or (F1+F3)−(F2+F4), is equal to that obtained before the astigmatism is produced, even if astigmatism is produced in the splitted sub-beams SBF1 and SBB1, and SBF2 and SBB2 as indicated by dotted lines. That is, there may arise a case in which a difference between a signal value produced from the E1 and E3 regions (or F1 and F3 regions) and a signal value produced from the E2 and E4 regions (or F2 and F4 regions) equals zero identically before and after the astigmatism is produced. Therefore, an error can be caused in an FE signal determined through the differential astigmatism method, whereby not only the accuracy of focusing control but also the reliability of the optical pick-up apparatus may be deteriorated due to such an error.
Accordingly, a need exists for an improved diffraction element for an optical pick-up apparatus that enables more accurate focusing control.